1. Field of the Invention
The present invention relates to a passive optical network, and more particularly to a passive optical network capable of controlling change of intensity of an optical signal due to transmission distance.
2. Description of the Related Art
Generally, a passive optical network (PON) includes an optical line termination (OLT) for providing a service, a plurality of optical network units (ONUs) using the service from the optical line termination, and remote nodes for connecting the optical line termination to the optical network units. In the passive optical network, the optical line termination is connected to the remote node through one optical fiber and the remote node is linked to the optical network units through additional optical fibers. The optical line termination generates downward optical signals to transmit to each optical network unit, and each optical network unit generates an upward optical signal to be outputted to the optical line termination.
In the aforementioned passive optical network, since distances between the optical line termination and each optical network unit are different from each other, the upward optical signals transmitted from each optical network unit to the optical line termination have intensities different from each other due to dispersion of the optical signals. Accordingly, the optical line termination must further include a burst-mode optical receiver for changing a discrimination critical value according to the intensity of each upward optical signal inputted from each optical network unit.
FIG. 1 is a block diagram showing a structure of a burst-mode receiver which can be applied to an optical line termination in a conventional passive optical network. The burst-mode receiver 100 includes a photodiode 110, a preamplifier 120, a first peak detector 130, a first limiting amplifier 140, a second peak detector 150, a second limiting amplifier 160, an envelope detector 180, a comparator 190, and a buffer 170.
The photodiode 110 converts an upward optical signal received from each optical network unit into electric current, and the preamplifier 120 converts the electric current received from the photodiode 110 into voltage and outputs the voltage.
The first peak detector 130 detects a top-peak value and a bottom-peak value of the voltage converted by the preamplifier 120 and calculates an intermediate value between the top-peak value and the bottom-peak value. The first limiting amplifier 140 differentially amplifies the voltage outputted from the preamplifier 120 and the intermediate value of the voltage calculated by the first peak detector 130.
The second peak detector 150 detects a top-peak value and a bottom-peak value of voltage provided from the first limiting amplifier 140, calculates an intermediate value of the two values, and provides the calculated intermediate value to the second limiting amplifier 160. The second limiting amplifier 160 differentially amplifies the intermediate value of the voltage detected by the second peak detector 150 and the voltage which has been differentially amplified by the first limiting amplifier 140 and provides the amplified voltage to the buffer 170.
The buffer 170 buffers the voltage differentially amplified by the second limiting amplifier 160 and outputs the buffered voltage and the envelope detector 180 detects an existence or absence of a packet signal with respect to the voltage outputted from the buffer 170. Further, the comparator 190 compares an output voltage of the envelope detector 180 with a reference voltage set in advance and generates an inverse reset signal having a polarity inverse to that of a reset signal.
An optical line termination having a burst-mode receiver must further include an additional circuit such as an automatic gain controller in order to satisfy reception sensitivity. In order to determine a discrimination critical value, increased elements must be considered in designing a passive optical network, for example, a charging condenser, etc., in a peak detector must be discharged by generating a reset signal in each optical network unit.